1. Field of the Invention
The present invention relates to an optical recording medium and an optical recording and reproduction method, in which the optical recording medium is illuminated by near-field light to perform recording and/or reproduction.
2. Description of the Related Art
An optical (or magneto-optical) recording medium represented by a CD (Compact Disc), MD (Mini Disc) and DVD (Digital Versatile Disc) is widely used as a storage medium for music information, video information, data, programs and the like. In a system recording to and reproducing from those optical recording media, an objective lens faces a surface of the optical recording medium in a noncontact manner to read minute recording marks by detecting minute concavity and convexity formed on a recording surface of the optical recording medium and detecting reflectivity modified structure of a phase-change material. In the case of a magneto-optical recording method, magnetic domain structure where a Kerr rotation angle is changed is detected to read minute recording marks.
In recent years, since a larger capacity and a higher density are desired in such optical recording media, studies have been made on technologies for forming further smaller recording marks in an optical recording medium and reading the recording marks with high resolution.
When λ is a wavelength of illuminating light and NA is a numerical aperture of a focusing lens to focus the light on an optical recording medium, the size of a light spot illuminating the optical recording medium is approximately λ/NA, and resolution is proportional to the value. The numerical aperture isNA=n×sin θwhere n is a refractive index of a medium, θ is an incident angle of a peripheral ray on an objective lens. When the medium is air, NA may not exceed 1 and there is a limit with respect to resolution. Hence, a wavelength of a light source which is, for example, a semiconductor laser has been made shorter and a numerical aperture of a focusing lens has been made larger in an optical recording and reproduction apparatus.
On the other hand, what is called a near-field optical recording and reproduction method of using an evanescent wave that is light exponentially attenuating from an interface is proposed as a method of obtaining a numerical aperture larger than 1. In the near-field optical recording and reproduction method, a gap between a focusing lens and the surface of an optical recording medium may need to be extremely small.
An optical recording and reproduction method using a solid immersion lens (SIL) is proposed as a method in which near-field light illuminates an optical recording medium to perform recording and reproduction (for example, refer to Patent Reference 1 and Non-patent Reference 1).
FIG. 1 shows a schematic constitutional diagram of an example of an optical recording and reproduction apparatus that uses SIL as a near-field light illuminating portion. As shown in FIG. 1, the optical recording and reproduction apparatus includes a light source 20, a collimator lens 21, a beam splitter 22, a polarizing beam splitter 23, a ¼ wavelength plate 24, an optical lens 25 and a near-field light illuminating portion 26 that is SIL in this case, which are disposed on the optical axis in this order. Further, a first light receiving portion 27 is disposed on an optical path of light reflected by the polarizing beam splitter 23, and a second light receiving portion 28 is disposed on an optical path of light reflected by the beam splitter 22. A dashed line C shows the optical axis.
In the optical recording and reproduction apparatus having such structure, light emitted from the light source 20 is made into collimated light by the collimator lens 21, and after passing through the beam splitter 22 and the polarizing beam splitter 23, a phase thereof is advanced by a quarter of the wavelength through the ¼ wavelength plate 24. Then, the light illuminates a recording surface of an optical recording medium 110 as near-field light through the optical lens 25 and the near-field illuminating portion 26 that is SIL, for example.
Return light from the optical recording medium 110 is incident on the polarizing beam splitter 23 through the near-field light illuminating portion 26, the optical lens 25 and the ¼ wavelength plate 24. Since the phase has been advanced by half the wavelength after passing through the ¼ wavelength plate 24 on a forward path and a return path, the return light from the optical recording medium 110 is reflected by the polarizing beam splitter 23 and is received by the first light receiving portion 27.
On the other hand, since the polarization is converted on the edge of SIL, return light totally reflected on the edge of the near-field light illuminating portion 26 that is SIL in this case is transmitted through the polarizing beam splitter 23 and is reflected by the beam splitter 22 to be received by the second light receiving portion 28.
Specifically, in the optical recording and reproduction apparatus shown in FIG. 1, the first light receiving portion 27 detects information recorded on a recording surface of the optical recording medium 110. On the other hand, the second light receiving portion 28 detects the totally reflected return light which changes depending on a distance between the optical recording medium 110 and the near-field light illuminating portion 26 facing the optical recording medium 110. Therefore, distance, that is, a gap between the surface of the optical recording medium 110 and the edge of the near-field light illuminating portion 26 such as SIL can be detected by the amount of return light detected by the second light receiving portion 28.
For example, an optical recording medium 110 of a phase-change recording type, which is used in the aforementioned recording and reproduction apparatus, is proposed as shown in FIG. 2. FIG. 2 is a constitutional diagram schematically showing cross-section of an example of the optical recording medium 110, in which a reflective layer 102 made of Al or the like, a dielectric layer 103 made of SiO2 or the like, a phase-change material layer 104 made of GeSbTe or the like, and a dielectric layer 105 made of SiO2 or the like are sequentially laminated on a substrate 101 made of glass, polycarbonate (PC) or the like. Further, an optical recording medium 110 of a read-only type, which is used in the aforementioned recording and reproduction apparatus, is proposed as shown in FIG. 3. FIG. 3 is a constitutional diagram schematically showing cross-section of another example of the optical recording medium 110, in which a concave-convex pit corresponding to recording information is formed on a substrate 101 made of glass, PC or the like and a reflective layer 102 made of Al or the like is formed thereon (for example, refer to Non-patent References 2 and 3).
In the case where the aforementioned near-field light illuminating portion such as SIL is used, distance between the surface of the near-field light illuminating portion and the surface of an optical recording medium, that is, a gap is desirably equal to or less than one tenth of a wavelength of illuminating light (for example, refer to Non-patent reference 4).
Therefore, in the case where the near-field light illuminating portion such as SIL collides with the surface of the optical recording medium, there is a possibility of causing damages on the part where information is recorded. In order to control or avoid such inconvenience, for example, a structure schematically shown in an cross-sectional structure of FIG. 4 is disclosed in which a protective layer 108 having the thickness of approximately 1 μm or more is provided on the uppermost surface of the information recording surface of the optical recording medium 110 (for example, refer to Non-patent reference 5). In FIG. 4, the same reference numerals are given to portions corresponding to those in FIG. 2, and redundant explanation thereof is omitted.
As shown in FIG. 4, a focal position of light applied by the near-field light illuminating portion 26 is set to be on the surface of the recording and reproduction layer, which is the surface of the phase-change material layer 104 in the example shown in the figure, through the protective layer 108. In addition, in this case also, the gap between the surface of the optical recording medium 110 and the surface of the near-field light illuminating portion 26 made of SIL or the like may need to be approximately one tenth or less of the wavelength of illuminating light.
[Patent Reference 1] Japanese Published Patent Application No. H5-189796
[Non-patent Reference 1] I. Ichimura et al., “Near-Field Phase-Change Optical Recording of 1.36 Numerical Aperture”, Japanese Journal of Applied Physics, Vol.39, pp.962-967(2000)
[Non-patent Reference 2] M. Shinoda et al., “High Density Near-Field Optical Disc Recording”, Digest of ISOM2004, We-E-03
[Non-patent Reference 3] M. Furuki et al., “Progress in Electron Beam Mastering of 100 Gb/inch2 Density Disc”, Japanese Journal of Applied Physics Vol.43, pp.5044-5046(2004)
[Non-patent Reference 4] K. Saito et al., “A Simulation of Magneto-Optical Signals in Near-Field Recording”, Japanese Journal of Applied Physics, Vol.38, pp.6743-6749(1999)
[Non-patent Reference 5] C. A. Verschuren et al., “Towards cover-layer incident read-out of a dual-layer disc with a NA=1.5 solid immersion lens”, Digest of ISOM2004, We-E-05